HICKOK: Model 800, Tube & Transistor Tester

As I’ve been working on valve amps for a while now, I decided it was finally time to get a proper tube tester.  You can absolutely get by without one - for a long time, I managed without.  But I use a lot of old valves that often come with the amp, or are pulled from vintage equipment - it sometimes felt like I was flying blind.  Some valves were clearly performing better than others, but without a tester I had no reliable way to work out what was going on.

There’s a huge range of testers out there. I even considered building one myself at one point.  Broadly speaking, they fall into two main categories:

Emission testers:  These are the simpler kind, and they measure cathode current.  You pop the tube in, adjust a few parameters, and a single meter tells you "bad  ?  good".   They’re handy for quickly screening dead or weak tubes, but don’t tell you much beyond that.  Some also test for shorts, which is very handy.

Mutual conductance (Gm) testers:  These are a step up from an emission tester.  Instead of just telling you if a tube works, they give you a measurement of its transconductance (how well it amplifies), which is a much better indicator of how the tube will actually perform in an amp.  They also often have the "good / bad" reading as well.   

HICKOK 800

Why the Hickok?

I went with a Hickok 800, one of the classic mutual-conductance testers. It’s not the absolute top of the range (those honours go to the lab-grade Hickoks and AVOs), but it’s a very capable and well-regarded unit. It also fit within my budget, had been recently serviced, and came with a reference valve with accurately measured values for calibration.  An added bonus was that it had been modified to allow cathode-current measurement using an external multimeter — a great feature for matching pairs.

Using the Hickok, there are three things that I test for.  Leakage, transconductance and gas.  

Leakage tests for resistance between the various elements in the valve.  If a short is found, the valve goes straight in the bin - no further tests required.  I valve with shorts is bad news, it can damage equipment.  

Gas - All vacuum tubes contain a small amount of residual gas, but if that gas becomes excessive - due to age, seal failure, or internal damage - it can cause the tube to conduct current even when it shouldn’t.

The Hickok checks whether the valve is pulling grid current when the grid should be at zero or negative potential. In a healthy valve, the control grid doesn’t draw measurable current. But if there’s too much gas inside, it gets ionized by the electron stream and starts to act like a leaky diode, drawing current from the grid circuit.  That’s a red flag.

Mutual conductance (often abbreviated Gm) is a measure of how effectively a tube controls plate current with its grid voltage. It’s expressed in micromhos (µmhos), which is the same as µA/V (or milliamps per volt).

A higher Gm means the tube is more responsive — i.e. a small grid voltage swing causes a larger change in plate current. This directly correlates to gain and overall performance in an amplifier.

The Hickok has a chart listing the average GM of new valves, under the test conditions that the Hickock uses.  As an example, a 12AX7 lists 1,250 GM so if a valve under test reads at 500, it's clearly in a bad way.  

The manual also suggests testing under adverse conditions, which involves dropping the heater voltage down a notch and seeing if it still performs well.  If it does, this is a sign that it has plenty of life left.

THE CONTROL PANEL

Clearly, there are a lot of buttons and knobs to contend with.  But it's really not that complicated.

The first place to start is with the roller chart, or the manual.  This is where you find test settings - and you will want to double-check these, as it's easy to miss one.   Most valve testers have some form of manual that lists settings for the same purpose - what's the pinout and test conditions.

LEAKAGE 

This is used to test for shorts or resistance between valve elements & to switch the meter into test mode.  

This is the first test to do on any valve.  The knob is turned from 5 down to 1, while watching the meter for movement.  Note the leakage scale on meter measured in kilohms and megaohms.  For good valves, the meter barely moves, if at all.   

After testing for leakage, move the leakage knob to the TUBE TEST position.

FILAMENT

This sets the heater voltage, which for pretty much everything I use, is either 5, 6.3 or 12.6v.  

The SELECTORS set the pinout of the valve, for heaters, grid, plate, screen, cathode and suppressor.  The buttons above are for various tests that can be performed.  

The LINE ADJUST knob is a mains voltage trimmer - line voltage from the wall can vary, and by pressing the line voltage button, the meter will show if it needs adjustment (note the line test mark on the meter).  The weird looking socket next to it is for straightening bent pins.  

BIAS & ENGLISH

The BIAS sets grid conditions; the ENGLISH knob actually sets meter sensitivity or range.  It's called the English, as it can be set to rate the valve on the "bad  ?  good" scale.   

I prefer to set the range to use the Micromhos scale, which is in three ranges.  0 to 3,000.  0 to 6,000  and 0 to 15,000.   There are red dots on the English knob to show where to set it for this purpose.  

Using the ranges mentioned above, a valves GM can be directly compared to known values.  

Analog Signal Generator

This is my scruffy old analog signal generator that I picked up for $15 or $20.   I was actually surprised it still worked, given the apparent age.

If you don't have a signal generator and you see one going cheap, I'd suggest buying it if you are going to be building a few FX pedals or playing with amps.  While it doesn't have all the bells and whistles of the new digital ones, it just sits there and kicks out a nice sine or the occasional square wave - I never actually turn it off.

There's something to be said for having a reliable reference signal that never changes when fault-finding or testing pedals.  Given that I'm a big fan of oscilloscopes for testing pedals, it goes without saying that a good signal generator is a must-have (at least for me, anyway).

For pedals, I always run it at 440hz, and 130mv TRMS.  While 1kHz is usually considered a standard test wave, it annoys me after a while.  As you probably realise, 440hz is actually the A string of a guitar.  

I use sine waves for testing most things, except for filters or equalisation that actually need harmonic content to filter, which is where I use a square wave.  

Sine waves are great for testing any kind of overdrive/fuzz/distortion, as you can actually see the harmonics generated by distorting the wave, as sine waves have no harmonics by definition, just the fundamental frequency.

These can also be used to work out the turns ratio of output transformers on valve amps, but that's a topic for another post.

VISUAL ANALYSER: Spectrum Analyser, Oscilloscope, Signal Generator

Thought I might share a tool that I've found very useful, which is not well known  -  Visual Analyser is a free PC-based oscilloscope / spectrum analyser / signal generator.  http://www.sillanumsoft.org/prod01.htm  

It's great for fault-finding the signal path or testing transistors for your effects.  example:  If two transistors both look identical on the scope and spectrum analyser, they will almost certainly sound the same.

I'm using iRig on an old laptop as a cheap and dirty interface, with some proper scope probes that I picked up for free.  If you have multiple inputs, it can be run as a dual scope.

Example:  Tone Bender MKIV

VISUAL ANALYSER - BASIC SETTINGS / HOW TO FOR GUITAR PEDALS

LIMITATIONS & SAFETY WARNING

Unlike a hardware-based oscilloscope, a sound card is not designed to accept high AC or DC voltages.  They are designed for small AC signals only - microphone and line levels.   So unless you want to destroy your computer and possibly electrocute yourself - stay away from high voltages as these may be lethal.  i.e. you may end up dead.

Make sure there's a capacitor protecting the sound card from DC voltages.  There should be one on the input of the sound card, but I like having one in the signal chain prior to this anyway (more on that later).

Frequency range.  Again this relates to the sound card.  A standard scope can measure frequencies in the MHz, while with a sound card, you will be lucky to make it to 20kHz.   This isn't necessarily a big deal for most analog guitar effect pedals, especially if you are only dealing with audio signals that fall within the usual range covered by a sound card.  What you will probably not be able to measure are waveforms generated for tremolos (very low) and clock rates for some digital ICs (very high).

MAIN WINDOW

After downloading and installing the software, the first thing you will see is the main window.

The main window is split between the scope view on the top and a spectrum analyser below.  There's also the option to float each view separately, so if you want to just have a large scope window, you can do that (see the floating button on the top toolbar).

I prefer to leave them running at the same time.  

At this stage, you will probably want to run some level into Visual Analyser to see what it can do.

 

WAVES AND SIGNAL LEVELS

A 1K sine wave is pretty standard, although I prefer 440hz, as I find it a little gentler on the ears.

Of course, nothing beats plugging a guitar into a circuit with an amp at the other end; it’s just not always practical.  Using a guitar on a scope is pointless, as the wave is very complex and constantly on the move.

Testing overdrives, distortions, fuzz etc

I always use a sine wave, as any circuit that distorts the wave, generates harmonics.  If you use anything other than a sine wave, the resulting wave becomes complex, making it harder to see and hear the results.

Testing equalisation or filters

Use a square wave, or pink / white noise.  Why?  both options have a lot of frequency content / harmonics that can be filtered or equalised.  It’s near impossible to equalise or filter a sine wave, as it only has one frequency (the fundamental).  Unless of course, the eq or filter is in circuit that is generating a lot of harmonics already, so you actually have something to filter and the results can easily be seen.  i.e. fuzz, distortion etc.

Signal Level

I use about 130mv TRMS as a signal level, which is about 360mv peak to peak.  200mv is fine, even 500mv for hot pickups.   Personally, I would not go over 1v, as you might start to get unexpected results.  i.e. driving the input too hard.   

A 1kHz sine wave at 1V is sometimes seen in service manuals as a test signal.   

SOUND CARD SETTINGS

Don’t max the gain on the test signal, as you will need a lot of headroom left on the sound card to cope with higher levels coming off the circuit during testing.  My input level sits at around -24db.  You can always zoom in on a small signal, but once the card clips / you run out of headroom, you can’t fix this.

Set the sound card to the highest sample rate possible, and adjust the buffer if needed.  Same as setting up for any audio application.

I actually have a voltage divider set up on the output of the probe before the iRig / sound card to help keep levels down.  A volume pot would also do the job, however, I prefer not having something that needs adjustment all the time.

Input chain:   Probe ---> 10uf cap ---> voltage divider ---> iRig ---> Computer sound card input

Monitor speaker:  Probe ---> 10uf cap ---> Line input of powered speaker

SPECTRUM ANALYSER SETTINGS (FFT)

I set the range to 200hz to 20kHz, as I use a 440Hz test signal.  No point in having half the analyser window empty, and this provides more room to see the harmonics.  I've used a square wave as an example here, as, unlike a sine wave, it actually has harmonics to see.

If you are new to the concept of harmonics - they are whole number multiples of the fundamental frequency, and they basically give sound character/complexity. 

There's also a 3D mode for the window that scrolls live.  Bit of a Joy Division vibe with this.   I think it's mostly pointless for my purposes, but it looks cool.   

Below is a great example of harmonics - this is an octave fuzz.  The fundamental frequency of 440hz is dwarfed by the second harmonic of 880Hz.  It also has a lot of high-frequency content that's quite pronounced.

I prefer to set the Y-axis scale to 3db (this is the vertical scale showing level).  For the 3D view, just check the button "3D" in the X-axis settings.

I also prefer not to have the window autoscale, as this can get confusing regarding changes in levels.  To manually change the level, click on the Y-axis scale and drag it up and down (the green bar on the right of the analyser window.

Note on the top right the buttons for capturing the scope or spectrum results.  It's basically a screengrab.  Very handy when comparing different parts or settings.  You can also access this via a right-mouse click anywhere in the window.

OSCILLOSCOPE SETTINGS

The scope is pretty simple.  I rarely touch anything apart from Zoom or ms/d.

Zoom - this is the vertical zoom used to increase or decrease the wave size.  I normally have mine sitting on 4, and then I zoom out to 1 for large signals.  

ms/d - or millisecond/division.  This is the time base or horizontal zoom.  It also helps steady the wave if you have a wave that is flickering across the screen.   The Trig slider also helps with this.

Vpos - this sets the vertical position of the wave, only needed if something is very asymmetrical or you want to position the two channels separately in the scope window.

Values - enabling the Values provides an extra display with a range of different values, most of which will probably not be of any interest for most people.  Give it a try; maybe you will use it.

DA checkbox - this is the digital conversion option.  I will not attempt to explain it all here - just turn it on.   Read the help file for a proper explanation.  TLDR - smoother-looking waves.  

DC removal checkbox - digital filter to cut frequencies below 0.01Hz

WAVE GENERATOR

If you don't have a signal generator, the wave generator might be handy (launch via the top toolbar).  There's not much the waver generator doesn't do - best to just start using it, I'm sure you will work it out.

Personally, I prefer a hardware option as a signal generator, which may not be practical for everyone.  I picked up an old analog one for $20 that is always on.